1. Field of the Invention
The present invention relates to a process for producing a polyacetal copolymer. In particular, it relates to an improved process for producing polyacetal copolymer being excellent in the qualities such as heat stability comprising simple steps, wherein the principal monomer is copolymerized with a comonomer copolymerizable therewith using a heteropoly-acid or an acidic salt thereof as the polymerizing catalyst.
2. Description of the Related Art
Conventionally, processes for producing cationic copolymers comprising trioxane as the principal monomer and a cyclic ether or cyclic formal having two or more vicinal carbon atoms as the comonomer have been known as the process for producing polyacetal copolymers. The cationic activating catalysts used as catalysts for these copolymerizations include Lewis acids such as halides of boron, tin, titanium, phosphorus, arsenic and antimony, e.g., boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentachloride, phosphorus pentafluoride, arsenic pentafluoride and antimony pentafluoride, and compounds such as complexes or salts of them; protonic acids such as perchloric acid; protonic acid esters such as esters of perchloric acid with lower aliphatic alcohols, e.g., tert-butyl perchlorate; protonic acid anhydrides such as mixed acid anhydrides of perchloric acid and lower aliphatic carboxylic acids, e.g., acetyl perchlorate; and trimethyloxonium hexafluorophosphate, triphenylmethyl hexafluoroarsenate, acetyl tetrafluoroborate, acetyl hexafluorophosphate and acetyl hexafluoroarsenate.
Among them, boron trifluoride and coordinate compounds of boron trifluoride with organic compounds such as ethers are the most conventional as catalysts for copolymerizing trioxane as the principal monomer with a comonomer and widely used industrially.
However, there have been problematic that conventionally used polymerization catalysts such as boron trifluoride series compounds are required in relatively large quantities (for example, 40 ppm or more based on the whole monomers), that the polymerization degrees of the resulting polymers are limited since the deactivating treatment of the catalyst is not sufficiently completed, even though the deactivating treatment is applied, and the decomposition is accelerated by remaining of substances originated from the catalyst after polymerization, and that the resulting polymers contain unstable terminal moieties in considerable amounts requiring complicated steps for stabilization thereof.
That is, in a process for copolymerizing trioxane by means of such conventional catalysts as described above, a deactivation of the catalysts after polymerization is important. While the deactivation thereof is insufficient, it accelerates a decomposition of a resulting polymer and largely causes an obstruction against a stability of the resulting polymer in later steps. Accordingly, when a boron trifluoride and the like are used as a catalyst, such very complicated steps that in order to sufficiently deactivate the catalyst, for example, a lot of a deactivator solution is added to the product obtained by the polymerization, the product is sufficiently washed to remove residual monomers and residues originated from the catalyst, and the deactivator solution is separated and dried, or the monomers have to be recovered from the washing solution. Such complicated steps are not preferred from an economical point of view.
Moreover, in order to eliminate complexity brought about by such deactivation treatment of catalysts, a method is proposed in which an addition amount of a deactivator solution is decreased and washing of a crude polymer is omitted (for example, JP-A-52-57285, JP-A-57-80414, JP-A-62-285909, and JP-A-63-27519). A method for deactivating the catalyst by contacting a gaseous deactivator to a resulting copolymer is further proposed (for example, JP-A-58-167608 and JP-A-2-263813). In such methods, however, such generally known polymerization catalysts as boron trifluoride series catalysts can not sufficiently be deactivated, and therefore it is extremely difficult to obtain polymers having a good heat stability. In particular, an increase in a polymerization yield can reduces the necessity for recovery and washing of monomers, but it makes a resulting polymer further unstable and requires a complicated stabilization treatment in a later step. Therefore, it does not result in simplification of the steps and restricts the stability of the products. Accordingly, it is not preferred in terms of quality.